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Prism is no longer developed by Mozilla Labs, although this documentation will still be useful to those using WebRunner (see next paragraph). There are now two alternatives:

The Prism code was picked up by Matthew Gertner who started a company to continue its open source development under the name WebRunner. See his blog post at http://www.salsitasoft.com/blog/2011/04/18/packaging-webrunner-applications/

Mozilla Labs itself started a new project called Chromeless. See their blog post at http://mozillalabs.com/blog/2011/02/prism-is-now-chromeless/